The visible spectrum of light extends from wavelengths of about 0.4 microns to about 0.7 microns. Wavelengths longer than visible wavelengths can be detected by dedicated sensors. However, SWIR light is reflective light which bounces off of objects much like visible light. As a result of its reflective nature, detected SWIR light can have shadows and contrast in its imagery. For low light level imaging applications, noise/leakage current such as dark current must be reduced to obtain sufficient sensitivity.
Sensors constructed from materials like mercury cadmium telluride (HgCdTe) or indium antimonide (InSb) can be very sensitive in the SWIR to MWIR band. However, at least in the case of HgCdTe, because of high dark current due to the narrow bandgap, these devices must be mechanically cooled, often to cryogenic temperatures, which increases power consumption, size and cost of cameras that utilize such sensors. Newer processes are making it possible to achieve SWIR to MWIR (1.1 μm-3.5 νm) imaging at higher operating temperatures, but SWaP (Size, Weight and Power) remain a challenge, as well, MWIR systems cannot image visible energy, such that systems requiring visible signal response (typically Blue 450 nm to red 700 nm) must also employ EO sensors for visible imaging and then merge the two datastreams with fusion algorithms or provide dual displays for managing the two separate bands.
Thermal imagers are another class of camera with good detection abilities. While thermal imaging can detect the presence of a warm object against a cool background, they provide low resolution and dynamic range across spectral content requiring sensing of reflected light, such as seeing objects like building, furniture, or other materials which are of minimal thermal deviation from the background scene. Additionally, due to the material properties of many glasses, particularly those utilized in industrial environments, LWIR (thermal spectrum typically 8 um to 12 um) is not transmitted, but is absorbed or reflected in the medium, thus thermal sensing alone is not capable of imaging through common window materials such as in buildings or ANSI Standard protective shielding in industrial assembly lines requiring such measures
Additionally, CMOS and CCD imagers are excellent devices that continue to evolve to meet military needs. But such sensors are typically utilized for visible light response, having near Infrared Response in the short-end of the IR spectrum (typically cutting off at no more than 1100 nm. In order to image well for night vision applications, based imagers require substantial amplification, which create image quality challenges due to noise amplification. Else, Silicon-based imagers are indirect bandgap, and while capable of being manufactured at low cost, with high resolution, suffer relative low sensitivity to NIR energy as compares to III-V direct bandgap materials with narrow absorption band to achieve higher sensitivity across the full SWIR band, such as Indium Gallium Arsenide.
InGaAs sensors can be made extremely sensitive (D*˜1E+14 Jones in commercial InGaAs imagers), literally counting individual photons. Thus, when built as focal plane arrays with thousands or millions of tiny point sensors, or photodiode sensor pixels, SWIR cameras will work in very dark conditions. Images from an InGaAs camera can be comparable to visible images in resolution and detail; however, SWIR images are not in color. This makes objects easily recognizable and yields one of the tactical advantages of the SWIR, namely, object or individual identification.
For conventional back-illuminated photodetectors, such as those which are used in hybrid IR Focal Plane Arrays structures, a back-illuminated imaging plane includes a substrate material of a higher bandgap than that of the active material. InGaAs for SWIR photodetection is traditionally grown on Indium Phosphide (InP) substrates, and is capable of photoresponse in both visible to SWIR bands (450 nm to 1700 mn). The InP (1.35 eV) substrate is of higher bandgap than the lattice-matched InGaAs (0.7 eV), this will limit useful application for InGaAs imagers to 1100 nm→1700 nm photoresponse unless the InP substrate is removed. The substrate, therefore, acts as a long-pass filter for all energy above that of its band gap. Wavelength is inversely proportional to energy (E=hc/λ, where: E is energy, h is planck's constant, c is speed of light, l is wavelength), thus longer wavelength passes above the absorption ‘cutoff’ of a given material. It is desirable, however, to extend the photo response of traditional compound semiconductor imaging in SWIR to include shorter (visible) wavelengths, especially for applications where a short-wave IR solution may displace those imaging devices having a true visible response incorporated therein.
In an approach to achieve visible response in a conventional InGaAs/InP based imaging systems, for example, the substrate is removed using mechanical polishing methods or the use of etch stop layers. However, because the substrate in such systems is commonly used as the cathode of the diode array (Substrate Contact), the substrate cannot be completely removed. Such solutions are presently offered as commercially, and for specialty applications.
In some material systems, as indicated in the above discussion, the substrate or buffer layer is of higher bandgap than that of the lower bandgap semiconductor active layer. In such systems, the lower bandgap semiconductors suffer high recombination velocity if p-n junctions are exposed at the surface, due to defects, surface states, and what is commonly referred to as carrier trapping. The defect states reduce the bandgap by providing reduced energy potential for conduction. Thus the absence of a high bandgap buffer layer (also known as cladding layer) degrades device performance. That is, the absence of an energy band offset in a region with low electric field does not allow efficient migration of photo-generated carriers (minority carriers). Accordingly, in the absence of a cladding layer, the quantum-efficiency suffers, due to minority carrier trapping.
Thus, conventional devices have relied upon a careful balance of thickness of remaining substrate “blocking layer” (buffer/cladding layer) to achieve desirable electrical properties such as limiting surface recombination, while also providing for adequate transmission of light to the active layer. One approach to minimize surface recombination in higher bandgap materials has been with N+ doping of cladding layers to assist in minority carrier migration. In such an approach, as the Fermi-level is moved closer to the conduction band in this layer, the result is carrier migration away from the surface and towards the junction. This explains the quantum mechanical dependency upon the outermost (exposed) layers of a semiconductor structure. While not limited to a particular theory, this applies equally to etched sidewalls as for the “front and back” surface of the wafer. Accordingly, it has become increasingly desirable to leave a sufficiently thick layer to maintain robust control over the electrical performance thereof and the quantum efficiency of the device. However, the optical absorption of the substrate layer requires that it be made substantially thin so as to allow transmission of visible wavelength light to the active layer. For example, a typical thickness of the blocking layer to achieve adequate visible response in SWIR imagers less than 1.5 μm. However, such thicknesses are vulnerable to etch-pits and various defects. To achieve robust electrical and mechanical performance, it is desirable to leave this layer as thick possible without cutting off photo response for the desired application.
One drawback of a conventional focal plane array, as shown in FIGS. 1A-1B in electrical communication with a read out integrated circuit 9, is with respect to the optical properties thereof. For example, as shown, the photodetector array includes a continuous backing layer 11 that transmits only radiation 22 having a first wavelength, such as infrared (IR) radiation, while completely blocking radiation 20 having a second wavelength, such as visible (vis) radiation. It would be desirable to have a focal plane array that overcomes these optical issues.
It is also important to note that some material systems do not have appropriate etch-stop layers for executing such a process as described above. For example, during fabrication of a conventional InGaAs sensor, selective etch stop layers are grown in order to allow for bulk substrate removal, such as InP substrate removal. As such, processes employed for those materials without chemical means of stopping substrate removal must utilize CMP or mechanical lathe or diamond turn operations. The thickness control of such methods is very good, however achieving <2 um thickness of remaining ‘blocking layer’ is not practical. The ability to leave the ‘blocking layer’ thicker would enhance the manufacturability of such structures.